1. Field of the Invention
This invention relates to an automatic transmission control system for an automobile which can control a torque draw caused at the beginning of, in particular, an up shift.
2. Description of the Related Art
Automatic transmissions for automotive vehicles, which typically comprise a torque converter and a transmission gear mechanism, are automatically shifted into desired gears by selectively locking and unlocking a plurality of friction coupling elements such as clutches and brakes to switch the torque transmission path of the transmission gear mechanism. This type of automatic transmissions are provided with hydraulic pressure control systems which perform control of supply hydraulic pressures to and discharge of the hydraulic pressures from the respective specific friction coupling elements to selectively lock and unlock them. In the hydraulic pressure supply control on a specific friction coupling element, precharge control is performed to make, for example, a duty solenoid valve as a pressure control means for controlling a working pressure to be supplied to the specific friction coupling element remain fully open for a certain time from an appearance of a gear shift command so as thereby to quickly fill a hydraulic pressure path leading to a pressure chamber of the specific friction coupling element with a working fluid.
The reason for performing the precharge control during a gear shift is that, in cases where locking or unlocking a friction coupling element is caused by supplying a working pressure to a pressure chamber of the friction coupling element, thee is no working fluid in a fluid line between a hydraulic control circuit and the friction coupling element at the beginning of a gear shift and a necessary level of fluid pressure is consequently not attained in the pressure chamber of the friction coupling element even when the working pressure is generated and supplied to the friction coupling element immediately after an appearance of a gear shift command, the locking or the unlocking of the friction coupling element is delayed. In one type of precharge control such as known from, for example, Japanese Unexamined Patent Publication No.1-303350, the precharge control is continued until an inertia phase of the control of a gear shift in which the turbine speed of the torque converter changes.
If an up shift is performed by locking a specific friction coupling gear in this type of automatic transmission, a problem of what is called a torque draw is encountered during a torque phase taking place at the beginning of the gear shift. The term of "torque draw" refers to the phenomenon of a temporary sharp drop in output torque in a period for which, while a torque change is caused due to the up shift, the turbine speed remains unchanged and leads to a shift shock which always provides aggravation of a gear shift feeling.
It has been known to make use of the precharge control in order to eliminate such a torque draw. For example, a prolonged precharge period or an increased flowing quantity of working fluid per unit time during the precharge control enhances a rise in the working pressure shortens the duration of a torque draw or the duration of a torque phase in which a torque draw potentially occurs, making the effect of a torque draw to the gear shift feeling less strong. However, simply increasing the precharge period or increasing the flowing rate of working fluid has adverse effects to the inertia phase after rising of the working pressure, or otherwise, in some driving conditions, results in aggravation of a gear shift feeling.